Update CONTRIBUTING docs for better PRs

- Focus on `site` over wiki.
- Ask for smaller pull requests.
- Ask for a description with motivations, what was done and any tasks.
- Use `bundle exec rake`
- Remove superfluous newline toward end of file.

Was talking with @maul-esel and thought it would be only fair to ask these things of our awesome contributors if we said it up-front. We want to make sure that contributions are manageable for our maintainers and still easy for our contributors. :smile: :tada: